1. Technical Field
The present invention relates to electrically stimulated filters which operate to remove particles, such as dust, from a fluid, such as air. More particularly, the invention relates to improved filtering and precharging in an electrically stimulated filter assembly.
2. Discussion of the Prior Art
Electrically stimulated filters are well known in the prior art. Examples of such filters may be found in the following: U.S. Pat. Nos. 2,973,054 (Kurtz); 3,242,649 (Rivers), 3,997,304 (Carr), 4,244,710 (Burger), 4,279,625 (Inculet, et al.), 4,313,739 (Douglas-Hamilton), 4,357,150 (Masuda, et al.) and 4,509,958 (Masuda, et al.); Canadian Patent Nos. 821,315 (Inculet) and 821,900 (Incultet, et al.); British Patent No. 892,908; Japanese Patent No. 52,37273; and German Patent Publication 25 32 727. Typically, in the filtering section of the filter assembly, prior art electrically stimulated filters employ electrodes which are in direct contact with the filter medium. This is best illustrated in the Masuda, et al., patents. The filter medium employed is electrically non-conductive and is typically a material such as fiberglass. The amount of current drawn by such electrically stimulated filters is reasonable when the gas to be filtered is at a low relative humidity. However, as the relative humidity of the gas increases, the high voltage current increases exponentially as illustrated by curve A in FIG. 13 of the accompany drawings. The ultimate result is either a drop in voltage across the filter unit or a total shut-off of the power applied to the unit. In either case the efficiency of the filter is drastically reduced. The result is unreliable filtering which is the main reason that electrically stimulated filter technology has not gained wide commercial acceptability.
Another problem area contributing to the lack of commercial acceptability of prior art electrically stimulated filters relates to the precharger. Prechargers are employed to electrically charge suspended particles in the gas, prior to the filtering stage, so that the charged particles may be more readily separated. A commonly employed prior art precharger, as disclosed in the above-mentioned Masuda, et al., patents, includes multiple grounded parallel plates with corona wires strung between them. This precharger design results in a high probability of error in achieving wires equispaced from grounded plates. If the wires are not equispaced from the grounded plates, current leaks through a local point resulting in severe reduction in ionization and, thereby, inefficient charging of the suspended particles by the precharger.